This invention relates to interleaved multiple signal separating circuits, and more particularly to a circuit for separating a video signal into a luminance signal and a chrominance signal in a color television or a color video tape recorder.
In the NTSC color television system, a chrominance signal is interleaved in the form of a carrier signal into the higher frequency band of a luminance signal. Therefore, it is necessary to separate the luminance signal and the chrominance signal from each other by using a band pass filter or the like; otherwise the two signals are interfered by each other, with the result that the picture quality is lowered.
In general, when a chrominance signal is separated from the video signal by a band pass filter whose central frequency is the carrier frequency of the chrominance signal, the higher frequency component of the luminance signal, included in the signal causes chromatic noise, i.e. cross color interference. If the carrier chrominance signal remains in the higher frequency component of the luminance signal, then dot interference is caused.
In order to relieve these interferences, the high frequency component of the luminance signal is reduced by the signal transmitting side, or the amplification degree of the higher frequencies in the luminance signal amplification system of a television set is reduced. However, these methods are disadvantageous in that the resolution is decreased.
Recently, research has been conducted to eliminate these interferences by using a comb filter with a solid ultrasonic delay line. As shown in FIG. 1, a video signal inputted through an input terminal 1 is delayed by one horizontal scanning period by a 1H delay line 2, and the video signal thus delayed is combined with the video signals which are applied directly (not delayed) through bypass circuits 3 and 4. As a result, the chrominance signal is provided at the output of a C type filter 5 which provides a difference signal of the two signals (the delayed signal and the not delayed signal), and the luminance signal is provided at the output of a Y type filter 6 which provides a sum signal of the two signal.
In this circuit shown in FIG. 1, the C type filter having a characteristic as shown in FIG. 2 is combined with the Y type filter having a characteristic as shown in FIG. 3 (a comb filter being formed), and therefore the video signal can be separated into the chrominance signal and the luminance signal.
In general, in order to satisfactorily operate a solid delay line such as a glass delay line, anti-resonate coils must be connected in parallel to the input and output transducers, respectively. In this case, the glass delay line shows a band pass characteristic in which the higher and lower frequencies are cut off. Accordingly, if the pass band of the glass delay line is set to a range whose center is the frequency band of the chrominance signal, then the lower frequency component of the chrominance signal is blocked in the circuit shown in FIG. 1, and accordingly it is necessary to provide a low-pass filter additionally to mix the lower frequency component with the luminance signal output in FIG. 1.
However, with such a mixing circuit, it is difficult to correctly phase signals, and the frequency characteristic is not flat. Therefore, a ghost-like ringing phenomenon attributing to the transient response of the ultrasonic delay line occurs in the picture. Accordingly, it is necessary to provide a more intricate circuit to prevent the ringing which leads to an increase in manufacturing cost. Thus, the circuit has not put to practical use yet.